The present invention relates to an image sensor used for example in an image reader.
FIG. 5 illustrates a conventional contact-type image sensor shown for example in Mitsubishi Denki Technical Journal. Vol. 60, No. 11, 1986.
As illustrated in FIG. 5, a linear light source 54 uniformly illuminates an original document 51 which is guided by an original document guide 52. A rod lens array 55 receives light reflected from the original document 51. The rod lens array 55 consists of a plurality of rod lenses and serves for formation of an erect image with one-to-one ratio. Light passing through the rod lens array 55 is focused on a linear sensor board 56 which includes an array of photodetectors, not shown. The light is converted by the photodetectors into electrical signals, and outputted to an external circuit, not shown. The linear sensor board 56 has the same width (dimension orthogonal to the direction of the relative movement of the original document 1 relative to the image reader). The linear light source 54 and the rod lens array 55 are accommodated in a housing 53.
FIG. 6A and FIG. 6B show another conventional contact-type image sensor shown in the Mitsubishi Denki Contact-type Image Sensor Catalogue (February 1989).
In this prior art image sensor, the original 51 is passed over a glass sheet 57, which is fixed to the housing 53 by means of side boards 59, only one of them being shown in FIG. 6B which is a sectional view of the rod lens array 55 as seen along the line 6B--6B in FIG. 6A. The side boards 59 are fixed by means of screws 60. The rod lens array 55 is supported by lens holder 58 in such a manner that its position along the direction of the light path can be adjusted. For this adjustment, a set screw 61 is turned, and a gap D is provided to allow the movement relative to the housing 53 for the position adjustment.
Light reflected from the original is passed through the rod lens array 55, focused to form an image on an array of photodetectors which are part of the linear sensor board. and converted into electrical signals representing the density or brightness at each area or pixels of the original document 1.
The electrical signals are transmitted to a signal detector, not illustrated, and electrical signals are thus obtained.
Used for the rod lens array, are Selfoc (trademark) supplied by Nihon Itagarasu K.K. FIG. 7 is a part of a explanatory diagram contained in the publication Nihon Itagarasu K.K. Technical Papers (SELFOC LENS ARRAY) printed in October 1987). The rod lenses 62 in FIG. 7A are made of glass which is a solid dielectric material. It has a distribution of the refraction index decreasing (graded) with the radial distance from the central axis as shown on the right side of FIG. 7A. Such light proceeds following a curved path with a certain pitch. The functional equivalent to that of a lens is thereby attained. The distribution of the refraction index is approximately given by the following relationship: ##EQU1## where n.sub.0 represents the refraction index at the central axis.
r represents the refraction index gradation coefficient, and PA1 r represents the distance in the radial direction from the central axis. PA1 r represents the distance from the central axis. PA1 n(r) represents the index of refraction at a location separated by r from the central axis PA1 n.sub.0 represents the index of refraction at the central axis and PA1 .sqroot.A represents the refraction index gradation coefficient.
FIG. 7B shows the structure of the rod lens array 55. The diameter of the rod lens 62 is normally about 1 mm and the range in which image is formed (radius of the field of vision) is about several mm. To obtain an image of a wide range, a multiplicity of lenses have to be used. As illustrated in FIG. 7B a multiplicity of lenses are arranged linearly in a frame 63 made of a glass epoxy resin having substantially the same thermal expansion characteristics as the lenses, and the interstices are filled with black rubber like silicone resin 64.
Conventional rod lenses 62 are made of glass, and to give them the refraction index gradation, wet ion exchange is conducted after spinning. The wet ion exchange however takes several days, and the process takes a long time. Because the process is of a wet type it is difficult to reduce the manufacture fluctuation in the refraction index gradation coefficient. .sqroot.A. As a result, the cost is high, and there is a fluctuation in the refraction index gradation coefficient, particularly between production batches. Consequently, the height Z.sub.0 of the rod lens array 55 had to be varied from one batch to another to obtain a constant conjugate length TC. With the type having an angular aperture of 20 degrees, which are most widely used, the range of the fluctuation is from +0.3 to +0.6 mm. Accordingly, conventional image sensors employing the conventional rod lens array is provided with the lens holder 58 and the set screw 61 for fine positioning of the rod lens array 55. The fine positioning of the rod lens array 55 in the direction of the optical axis is achieved by the rotation of the set screw 61 and the resiliency of the nails 58a of the lens holder 58 see FIG. 6B. Thus, the number of the parts required is large, and the work of the fine positioning is time consuming.